VSTHRD109 Switch instead of assert in async methods

Methods that are or can be async should switch to the main thread when necessary rather than throw an exception if invoked from a different thread. This allows callers to invoke any async method from any thread without having to concern themselves with the threading requirements of a method that can support its own threading requirements by switching.

Examples of patterns that are flagged by this analyzer

async Task FooAsync() {
    ThreadHelper.ThrowIfNotOnUIThread();
    DoStuff();
    await DoMoreStuff();
}

Solution

Use await SwitchToMainThreadAsync() instead:

async Task FooAsync() {
    await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync();
    DoStuff();
    await DoMoreStuff();
}